The invention relates to a wheel unit for a system for monitoring the wheels of a motor vehicle for errors states, in particular for incorrect tire inflation pressures. Furthermore, the invention relates to a method for activating a wheel unit which is assigned to a wheel of a motor vehicle and is a component part of a system for monitoring the wheels of a motor vehicle for error states, in particular for incorrect tire inflation pressures.
The term error states is to be broadly interpreted in the present context and covers all states which are considered worthy of detection.
The wheel units of the generic type and the methods of the generic type are used for example in connection with systems and methods for monitoring the wheels of a motor vehicle for error states, in particular for incorrect tire inflation pressures. In this connection, it is known for example that the wheel units transmit information by high-frequency signals to a central evaluation device, which is arranged on the chassis of the motor vehicle. In the present connection, a wheel unit is to be understood as meaning any unit which rotates with a respective wheel and is suitable for supplying information or data by means of which error states possibly occurring at the wheel can be detected. In addition to the actual detection of an error state, it is generally provided that the wheel position (for example front left, front right, rear left, rear right), at least of the wheels at which an error state has occurred, is determined, in order that the error state can be indicated together with the respective wheel position.
For example, EP 0 806 307 B1 already discloses a method for carrying out the assignment of the wheel position to air pressure control devices in an air pressure control system of a motor vehicle, the motor vehicle having, inter alia, the following component parts: a number of wheels, each wheel being assigned an air pressure control device, which is capable of emitting an individual identification to a central unit, and a central unit, in which the assignment of the identification of an air pressure control device to the wheel position of each wheel is stored. In the case of the method known from EP 0 806 307 B1, the assignment of the air pressure control device to the wheel position is performed as follows: switching on of an assignment mode; emission of the individual identifications of the air pressure control device; evaluation in the central unit of which individual identification was sent from which wheel position; storing of the determined assignment in the central unit; and switching off of the assignment mode and operation of the air pressure control system in a pressure monitoring mode. For the evaluation of individual identifications, it is in this case envisaged in particular to measure a specific variable, for example the rotational wheel speed, for each wheel of the motor vehicle both with a wheel unit of the air pressure control device and with a fixed sensor with a known wheel position. Subsequently, by a comparison of the output signals of the fixed sensors and the output signals of the wheel units, the wheel position of the individual wheels is concluded.
It is known, furthermore, from DE 197 20 123 C2 to provide in the region of each wheel to be monitored a dedicated antenna, with which signals emitted by the wheel units can be received. Although each of the antennas can under some circumstances also receive signals from wheel units which are not directly adjacent to the respective antenna, according to the teaching of DE 197 20 123 C2 it is adequate for determining the wheel position of individual wheels to assume that a received signal emitted by a directly adjacent wheel unit has a higher signal level than a signal which is received from a wheel unit arranged further away.
The known systems and methods share the common drawback that they require considerable expenditure on hardware and software and are therefore expensive.